James Cullen, the first Kid Quantum, was retconned into the Legion's past, ostensibly as a result of a history-changing disaster.[volume & issue needed] He was described as having been the first Legionnaire to die in action, as well as the reason for the Legion's rule against members whose superhuman abilities were solely technological; however, it was later revealed that "James Cullen" was a constructed identity used by a cluster of shapeshifting amoeboid "Proteans", who were seeking to protect their race, and that, instead of being killed, Cullen had simply gone into hiding until such time as it was safe. This plotline was never fully resolved, as the Legion's universe was subsequently "rebooted" with the Zero Hour mini-series.

The second Kid Quantum was also named James Cullen; however, this one was an actual person rather than a constructed identity. Like his previous counterpart, he used a so-called "stasis belt" to augment his power to freeze time within a limited area to useful levels. He became a member of the Legion of Super-Heroes and became its first member to die between his overconfidence and a belt malfunction. His death also caused the "no technology powers" rule to be implemented when later characters were recruited.

The third Kid Quantum was James' sister Jazmin. After her brother's death, she underwent surgery to augment her power to a level where she would not require the belt. Despite initial hostility to the Legion when she first joined, she was eventually elected leader of the team, thanks in large part to Cosmic Boy's recommendation of her; she later entered into a relationship with him. Jazmin is a native of the planet Xanthu.

Following the Infinite Crisis, the Multiverse was reborn, now with 52 alternate Earths. In the recent Countdown: Arena mini-series, Monarch, formerly Captain Atom, began assembling a team made up of alternate versions of heroes from various Earths. In the final issue, he killed every single alternate version of himself in order to gain their power. James Cullen was seen among the various Captain Atoms.